I don't like to "imagine" anything and would prefer to
leave these types of baseless accusations out of what
could potentially be an interesting and friendly
debate.

Haplogroups and haplotypes do go extinct. The aDNA
studies on mtDNA haplogroup N1c are a perfect example
of a lineage in high frequency in the Neolithic that
is rare to non-existent today among Europeans. The
aDNA study on a Cro-Magnon skeleton came back M*.
Clearly, this is another case of a lineage present in
Europe as early as 24,000 years ago, but relatively
non-existent among Europeans today? You would
therefore need to argue that the Y haplogroups have
experienced something all that different in terms of
genetic extinction, natural selection, founder
effects, etc., than that experienced by the mtDNA
haplogroups over the last 20,000 years.

What is the genetic evidence that the earliest
inhabitants of the British Isles (dating to
approximately 10,000 BC or so) belonged to R1b?

Furthermore, what is the genetic evidence that later
invader groups covering a period of nearly 9000 years
also were predominently R1b? And that we can't tell
the difference between R1bs from the Paleolithic from
R1bs from the Bronze Age, over 5000 years later?

The genetic evidence indicates that there was
significant impact from later mtDNA gene flow from the
Neolithic and later periods. If you are stating that
the Y haplogroups like R1b did not experience a
similar impact, what would lead to this difference
between Y and MtDNA groups over time?